In communication systems, such as a code division multiple access (CDMA) communication system, a mobile station and a base station establish a two way communication link through a forward and reverse communication links. The forward communication link originates from the base station, and the reverse communication link from the mobile station. The base station normally communicates simultaneously to a number of mobile stations. Accordingly, in a CDMA communication system, the forward link signals targeted for reception by the mobile stations are combined after channel encoding, interleaving and adjusting power level of each forward link signal. An example of such a system is described in "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systems" of the Telecommunications Industry Association/Electronic Industry Association Standard 95B (TIA/EIA/IS-95-B). To obtain a copy of the standard, EIA/TIA can be contacted at 2001 Pennsylvania Ave. NW Washington D.C. 20006. Through a power gain or voltage gain setting, the power level of each forward link signal in the combined signal is adjusted.
According to the protocol standard, the combined signal and each forward link signal individually may additionally be encoded for spreading the spectrum. Such encoding normally has a fixed encoding rate and is set according to the standard. The rate of the channel encoding which normally is dependent on the speech encoding rate of the channel, however, may be selected from a number of predefined encoding rates; the rate of the channel encoding hereinafter is referred to as the encoding rate. The combined forward link signal is amplified through a linear power amplifier before being transmitted from an antenna at the base station. The number of forward link signals that can be combined in the forward link communication defines the forward link communication capacity of the communication system.
The forward link communication capacity is limited by a forward link coverage limitation or a forward link interference limitation or both. The forward link coverage limitation is a geographical coverage limitation, and is a function of the maximum power level of the amplified combined signal. The maximum power level of the combined signal is set by the maximum allowable effective radiated power (ERP) mandated by the governing body for the signaling medium used (e.g. frequency spectrum) in the communication system. To reduce the limitation of the forward link communication capacity caused by the forward link coverage limitation, over a wide area serviced by a number of cells or sectors, the maximum allowable amplifying power of the linear power amplifier associated with each cell or sector is fixed to a predetermined level such that the ERP of the sector or cell is not violated, and as such, a geographical coverage area is adequately provided. Furthermore, to maintain an overall balanced coverage of the communication system, the maximum allowable transmit power level of all cells or sectors are adjusted coherently.
The forward link interference limitation, however, can not be remedied simply by adjusting the amplifying capacity of the linear power amplifier because the forward link interference limitation is due to interference caused by other forward link signals transmitted from adjacent or nearby sectors and cells of the communication system. The multipath effects of the forward link signals transmitted from the sector serving the mobile station and other sectors and cells are some other sources of interference. The forward link interference limitation forces the forward link signals to be transmitted at higher power levels for the mobile stations in certain "hot" areas within the coverage area that are effected by the interference limitation. Normally, there are higher concentrations of mobile users in the hot areas than the surrounding areas. As a result, the interference effects of the forward link signals in the hot areas are more pronounced than the surrounding areas. To increase the power level of a forward link signal, voltage gain or power gain of the forward link signal is adjusted to a higher than normal level.
When the forward link signals targeted for the hot areas require higher gain, the linear power amplifier would have less remaining linear amplifying power capacity. As a result, the total number of forward link signals that can be combined in the forward link communication is reduced. The combined effect of the gains of the forward link signals that are combined for amplifying through the linear power amplifier establishes the loading level of the power amplifier. The loading of the linear power amplifier should not exceed the maximum allowable amplifying power. Moreover, the amplified signal should not exceed the mandated maximum ERP level set by the appropriate governing body for that signaling medium. The loading level should be kept at a level that results in proper linear amplification of the combined signal. Since the number of forward link signals that are combined defines the forward link communication capacity, the forward link interference limitation, thus, reduces the communication system forward link communication capacity.
Furthermore, when the combined signal has reached its maximum power level, to maintain linearity of the power amplifier operation, any increase in gain of any forward link may force other forward link signals to be involuntary dropped due to the resulting poor frame error rate. When the calls are dropped involuntary, the communication system reliability and efficiency are highly undermined.
Any adjustment of the maximum allowable amplifying power of the linear power amplifier at each sector or cell can not overcome the forward link interference limitation in the hot coverage areas because such adjustment effects a wide coverage area; such adjustment may also create new hot coverage areas. Moreover, any adjustments of the maximum allowable amplifying power of the linear power amplifiers of less than all sectors or cells would upset the balanced coverage.
Therefore, there is a need for controlling the forward link communication capacity in response to the forward link interference limitation to maximize the forward link communication capacity, and prevent involuntary dropping of the calls.